1. Field of the Invention
The present invention relates to an X-ray diagnosis apparatus which performs bolus DSA (Digital Subtraction Angiography) imaging and stepping DSA imaging of a subject.
2. Description of the Related Art
An X-ray diagnosis apparatus performs DSA imaging. In DSA imaging, a mask image is subtracted from a contrast image to generate a subtraction image with a contrast-enhanced blood vessel portion being emphasized. A contrast image is an image acquired during the inflow of a contrast medium. A mask image is an image acquired before the injection of the contrast medium. In bolus DSA imaging and stepping DSA imaging, for example, the X-ray diagnosis apparatus acquires a plurality of mask images while intermittently moving the C-arm or the bed along the body axis direction of a subject. The X-ray diagnosis apparatus then generates a long mask image by combing the plurality of mask images. The X-ray diagnosis apparatus acquires a plurality of contrast images at the same imaging positions as those of the mask images while moving the C-arm or the bed in the same manner. The X-ray diagnosis apparatus generates a long contrast image by combing the plurality of contrast images. The X-ray diagnosis apparatus then generates a long subtraction image by subtracting the long mask image from the long contrast image.
In this method, the relative positional relationships between X-ray focuses at the time of the acquisition of the respective mask images (or the respective contrast images) vary. A relative positional relationship indicates, for example, X-ray focus positions relative to a subject. Such different positional relationships will make geometric enlargement ratios of the respective images differ from each other. Due to the differences in geometric enlargement ratio, pieces of information in the thickness direction of the subject interchange on the images. For example, as shown in FIG. 17, regions Pa and Pb at different positions in the thickness direction interchange on the images. That is, the positional relationship between points Pa1 and Pb1 extracted on an image I1 concerning an X-ray focus position F1 interchanges with the positional relationship between points Pa2 and Pb2 extracted on an image I2 concerning an X-ray focus position F2. Note that the points Pa1 and Pa2 originate from the region Pa, and the points Pb1 and Pb2 originate from the region Pb. It is theoretically impossible to accurately combine images with interchanged positional relationships.
As disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2004-242928, there is available a technique of acquiring images while spatially fixing an X-ray focus position, changing an X-ray irradiation direction, and moving an X-ray detector. Using this technique prevents pieces of information in the thickness direction of a subject from interchanging on images. In other words, a geometric enlargement ratio does not change. As shown in FIG. 18, for example, consider images I3 and I4 with the same X-ray focus position and different X-ray irradiation directions. In this case, the positional relationship between points Pa3 and Pb3 on the image I3 does not interchange with the positional relationship between points Pa4 and Pb4 on the image I4. Note that the points Pa3 and Pa4 originate from the region Pa, and the points Pb3 and Pb4 originate from the region Pb. It is therefore possible to easily combine the images I3 and I4.
In DSA imaging as well, using the conventional method makes it necessary, for the following reason, to acquire mask images and contrast images at the same imaging position. When an X-ray focus position concerning a mask image differs from an X-ray focus position concerning a contrast image, the information of a subject thickness on the mask image differs from that on the contrast image. In other words, the geometric enlargement ratio of the mask image differs from that of the contrast image. Along with this difference, the positioning accuracy between the mask image and the contrast image degrades, and the difference accuracy between the mask image and the contrast image degrades. This makes it necessary to obtain a mask image and a contrast image at the same position.